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International Journal of Ambient Energy
ISSN: 0143-0750 (Print) 2162-8246 (Online) Journal homepage: https://tandfonline.com/loi/taen20
Experimental investigation on the performance of
modified single basin double slope solar stills
S. Joe Patrick Gnanaraj & V. Velmurugan
To cite this article: S. Joe Patrick Gnanaraj & V. Velmurugan (2019): Experimental investigation
on the performance of modified single basin double slope solar stills, International Journal of
Ambient Energy, DOI: 10.1080/01430750.2019.1636861
To link to this article: https://doi.org/10.1080/01430750.2019.1636861
Accepted author version posted online: 26
Jun 2019.
Published online: 03 Jul 2019.
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3. 2 S. J. GNANARAJ AND V. VELMURUGAN
conventional single basin still. Tanaka (2013) theoretically anal-
ysed a tilted wick solar still and found that the daily productivity
of the still could be increased by using flat plate bottom reflec-
tors. Kalidasa Murugavel and Srithar (2014) studied the perfor-
mance of a single basin double slope solar still theoretically and
experimentally. The production was higher during the months
of March, April, August, November and December. The average
production was 2.1 L/day/m2. Arunkumar Singh, Rai, and Sachan
(2014) analysed the efficiency of the double slope solar still and
found that the energy efficiency of the still with south glass cover
was higher than that of the north side glass cover. Rajaseeni-
vasan, Raja, and Srithar (2014) concluded that the production
rate was high when jute cloth and black gravel were used in
the basin. Omara, Kabeel, and Younes (2014) improved the pro-
ductivity of the stepped solar still by 125% by using internal
and external reflectors. Rajaseenivasan, Kalidasa Murugavel, and
Elango (2015) investigated the performance of a double basin
still with different materials in the basin. They concluded that
the presence of mild steel pieces stored thermal energy in the
morning and released it during evening hours and it increased
the night production. Panchal (2015) found that the produc-
tivity in a double basin solar still increased by 9% when black
granite gravel was used and by 65% when black granite gravel
with vacuum tube was used. Srithar et al. (2016) used fins filled
with river sand and fins filled with charcoal in a triple basin
solar still. The presence of charcoal and river sand enhanced
the daily distillate by 34.1% and 25.6% respectively compared
to the conventional system. Rajaseenivasan et al. (2016) placed
energy storage materials (charcoal, sand and metal scraps) in
the hollow space of glass fins. The river sand, metal scarp and
charcoal enhanced the productivity by 26.74%, 29.3% and 33.7%
respectively. Kabeel and Abdelgaied (2016) used phase change
material (paraffin wax) as a thermal storage medium and the
productivity increased by 67.18% compared to the conventional
solar still. Deshmukh and Kolhe (2016) found that maximum
output could be obtained by connecting reflectors with evacu-
ated tube. Omara et al. (2016) investigated that the corrugated
solar still with wick and internal reflecting mirrors increased the
productivity by about 145.5% over the conventional solar still.
Madhua et al. (2018) conducted experimental study on the effect
of different sensible heat energy storage materials and found
that the rubber mat and polyester mat improved the yield by
57.1% and 59.5% respectively. Kabeel et al. (2018) studied the
productivity of solar still with and without jute cloth knitted with
sensible heat storage materials and found that the productivity
was 5.9 and 5.5 kg/m2 respectively. Panchal et al. (2018) found
that solid fins attached to the basin of the double basin solar
still increased the output by 25%. Mohammad et al. (2018) con-
ducted an experimental study on a solar still incorporated with
Sodium Thiosulphate Pentahydrate as PCM and connected to
an external solar collector. The unit was capable of producing
4300ml/day.m2,ofwhichabout4 0%wasproducedaftersunset.
Mousa et al. (2018) developed a theoretical modal to simulate a
solar still connected to an external solar collector and incorpo-
rated with sodium thiosulphate pentahydrate as phase change
material (PCM). Incorporating large PCM mass in the system
reduced the productivity. Increasing the ratio of PCM mass to
water mass from 10 to 100% reduced the productivity by up to
30%. Saadi et al. (2018) proposed to improve the performance of
conventional basin type solar still by integrating an internal ver-
tical multi-tray evaporator. Compared to the conventional solar
still, the increase in the daily output of the stepped solar still was
47.18%, 62.73%, 94.21% and 104.73% for spring, autumn, winter
and summer respectively.
In the above studies, the main thrust was on investigating
the impact of modifications on the quantum of distillate col-
lected. There is much difference in the response of the modified
solar stills for changes in the solar radiation intensity. To what
extent the solar still is sensitive to the solar radiation depends
on the modifications incorporated in the still. So, a performance
appraisal of the stills during the increasing and decreasing sun-
shine hours is necessary. Further, comparing the experimental
results obtained at different dates with different environmental
conditionsmakescomparisonmeaningless.Sothereisanurgent
need to track the performance of the still during the increasing
anddecreasingsunshinehoursafterprovidingidenticalenviron-
mental condition to all the stills.
In the present study, more emphasis was given to study the
rate of change of temperature parameter values and production
values during the increasing and declining sunshine hours. By
conducting the experiments in a single day in the conventional
and stills with various modifications, the drawback of compar-
ing the experimental results obtained at different environmental
conditions was eliminated. In addition to this, for an in-depth
study of the production pattern of the stills, the period of exper-
iment was divided into four quarters and the performance of
the stills in each quarter was compared. Further, an hour to
hour comparison of the surplus or shortage of production of the
modified stills over the conventional still was also done.
The solar radiation intensity is high at noon. Hence, the dis-
tillation output should be more during this period. Due to the
incident of higher amount of solar rays into the basin, the basin
water attains maximum temperature at about 2 pm. As a con-
sequence, the water vapour formation is accelerated during this
period.Theinnerglasscovertemperaturealsoincreasesbecause
of the higher temperate of air + vapour mixture in the still basin.
The distillate output is determined by the temperature differ-
ence between basin water and inner glass cover. The prevalence
ofhigherbasinwatertemperatureandtheinnerglasscovertem-
perature (low water-glass temperature difference) restrict the
distillate production rate. The presence of heat storage mate-
rial in the basin helps in the absorption of excess heat energy
received when the solar intensity is high and facilitates the
release of the stored energy to basin water, when solar intensity
is low. This helps in sustaining higher distillate production rate
during the declining sunshine hours and off sunshine hours
(night time).
The researchers improved the distillate production of the still
by spreading heat storage materials like black granite, charcoal,
pebbles, black rubber, brick stone, concrete pieces, sand and
metal scrap in the still basin or by filling the fins with these
materials. In the present study, instead of using one heat stor-
age material, 5 different materials were used at a time. The heat
storage materials that were spread at the bottom of the basin
were – black granite pieces, red brick pieces, pebbles, char-
coal and sand. The still basin was divided into 25 segments and
the basin materials were spread in the segments as shown in
Figure 1.
4. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 3
Figure 1. Arrangement of heat storage materials in the basin.
Figure 2. Still I. Conventional type still - Schematic diagram.
If black granite occupied one segment, the other basin mate-
rials occupied the other four neighbouring segments.
Further, the researchers used internal and external reflectors
in the still to improve the productivity of the still. So, in the
present study an attempt was made to enhance the distillate
production of the still with the help of external reflector fitted on
a adjustable metal stand. The length and width of the reflector
was 100 and 40 cm respectively. To focus maximum solar radi-
ation into still basin, the direction and angle of the mirror was
periodically adjusted. The objective of the study was to estimate
the efficiency of heat storage materials and external reflector in
improving the performance of single basin double slope solar
still.
2. Experimental setup
For investigating the above objective, three single basin double
slope solar stills with identical dimensions were fabricated. The
schematic diagrams of the three stills are shown in Figures 2, 3
and 4.
The solar still I was a conventional type still. It was free from
any internal or external modifications. In solar still II, the heat
storage materials were spread at the bottom of the still. External
mirror fitted on a frame was used in the solar still III. Thus three
solar stills were designed.
1. Still I – Conventional type still
2. Still II – Still with heat storage materials
3. Still III – Still with external reflector
The actual experimental setups used in the experiments are
given in Figure 5(A–C). The important dimensions of the stills are
summarised in Table 1.
The length, width and height of each basin were 100, 60
and 40 cm respectively. They were made up of 2 mm thick iron
sheet. Each basin was placed inside a wooden box and a gap
of 1.5 cm was maintained between the basin and wooden box
inner wall. The gap was tightly packed with saw dust. The inner
surfaceofthestillbasinwaspaintedblacktoincreasetheabsorp-
tion of solar radiation. Each basin was covered by a double
slope glass cover. The basin cover was made up of 4 mm thick
transparent glass and an angle of 30° was maintained. For col-
lecting the condensed water, drains were fabricated on the two
sides of the still. The distilled water was collected in a jar. Each
still was connected to the water source through a pipe for peri-
odic top up of water. The water supply was controlled by control
valves.
Severalinstrumentswereusedtomeasurethevariousparam-
eter values. The basin water temperature and inner glass cover
temperature was measured using k-type thermocouples hav-
ing the least count of 0.1°C. Thermocouples were connected to
a digital temperature indicator. A calibrated glass jar of 1000
ml capacity (0–1000 ml) with accuracy of ± 10 ml was used to
measure the hourly distillate collected. The solar radiation was
measured with the help of Kipp-Zonen solarimeter. Ambient
5. 4 S. J. GNANARAJ AND V. VELMURUGAN
Figure 3. Still II. Still with heat storage materials - Schematic digram.
Figure 4. Still III. Still with external reflector - Schematic digram.
Figure 5. Single basin double slope solar stills. (A) Conventional still, (B) Still with
reflector and (C) Still with heat storage materials (internal view).
temperature was measured with the help of a thermometer.
Alcohol in glass thermometer was used.
3. Mathematical modelling
To find the production of conventional and modified stills theo-
retically, the methodology used by Rajaseenivasan and Kalidasa
Murugavel (2013) was used.
Evaporative heat transfer (water to glass)
Qe,w−g = he,w−g(Tw − Tg) (1)
Evaporative heat transfer coefficient (water to glass)
he,w−g = 16.273 × 10−3
× hc,w−g
(Pw − Pg)
(Tw − Tg)
(2)
Saturated pressure for water
Pw = exp 25.317 −
5144
(Tw + 273)
(3)
Saturated pressure for glass
Pg = exp 25.317 −
5144
(Tg + 273)
(4)
Convective heat transfer coefficient (water to glass)
hc,w−g = 0.884 (Tw − Tg) +
(Pw − Pg)(Tw + 273)
268.9 × 103 − Pw
1
3
(5)
Calculation of daily productivity
6. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 5
Table 1. Important dimensions of double slope single basin solar still.
S. No. Particulars Dimension
1. Basin – Still I, Still II and Still III
Length 100 cm
Width 60 cm
Height 40 cm
Area 0.6 m2
2. Basin wall – Still I, Still II and Still III
Material Iron sheet
Thickness 2 mm
3. Basin cover – Still I, Still II and Still III
Type Double slope
Material 4 mm thick glass
Angle of inclination 30o
4. External reflector – Still III
Length 100 cm
Width 40 cm
5. Heat storage materials – Still II
Material Size Quantity
Black granite 10–15 mm 5 kg
Sand – 3 kg
Charcoal 15–20 mm 1.5 kg
Red bricks 15–20 mm 3 kg
Pebbles 10–15 mm 4 kg
The amount of distillate output of the solar still is
me =
Qe,w−g × 3600
L
ml/m2
(6)
L – latent heat of vapourization = 2,372,000 J/kg
The daily production of solar still is
yeildper day =
6pm
7am
me (7)
The daily efficiency of the solar still is as follows (Bhupendra
Gupta, Kumar, and Baredar 2017)
ηstill =
Me × L
I × A × 3600
(8)
where
Me = hourly productivity (ml),
I = hourly solar radiation (W/m2)
A = Area of basin (m2)
The efficiency of conventional still, still with heat storage
material and still with refector was 43.44%, 53.71% and 69.23%
respectively.
The initial ambient temperature is taken as the initial water
temperature, basin temperature and glass temperature. The
hourly variation in solar radiation and ambient temperature is
also recorded. The change in basin temperature ( Tb), water
temperature ( Tw) and glass cover temperature ( Tg) are
noted. By using the Microsoft excel program, the equations are
solved.
1. Energy gained by solar still I = The transient energy rise
from direct heating of sun.
2. Energy gained by still II = The transient energy rise from
direct heating of sun + Energy released from heat storage
materials.
3. Energy gained by still III = The transient energy rise from
direct heating of sun + Energy received from reflecting
mirror.
4. Results and discussion
The experiments were conducted during the months of March
and April 2016 in Villianur, Pondicherry, India. The experiment
was started at 7 am and continued upto 6 pm Before the experi-
ment was started, 5 l of saline water was poured into each still.
The top up of water was done at the end of every one hour.
The quantity of saline water added was equal to the per hour
distillate water collected from the still.
4.1. Impact of modifications on distillate production
The productivity of a solar still is influenced by various factors
such as ambient temperature, solar radiation intensity, wind
velocity, solar basin area, free surface area of water, water-glass
temperature difference, initial water temperature (inlet water
temperature), angle of the basin cover and water depth in the
basin (Velmurugan and Srithar 2011b). Among the above fac-
tors, ambient temperature, solar radiation intensity and wind
velocity are natural forces and human beings have no control
over them. The distillate production in a still is positively influ-
enced by ambient temperature and solar radiation intensity and
negatively influenced by wind velocity that prevails during that
particular day. Comparison of the modified solar stills with the
conventional still will be meaningful only when the experiments
in the three stills are conducted under identical environmen-
tal condition. To overcome the above problem mentioned, the
experiments were conducted in all the three stills simultane-
ously on the same day. Further to overcome the day to day
fluctuation in production due to environmental factors and to
get a vivid picture about the per day productivity of the stills,
the experiments were repeated for seven days and the average
of seven days production was taken as the per day production of
the stills.
The maximum ambient temperature recorded in a day and
the total distillate collected from the stills from 7 am to 6 pm is
given in Table 2.
The basin area of the conventional still and the two modified
stills is the same (0.6 m2). The maximum ambient temperature
recorded during the seven experiment days varied from 32°C
to 38°C. In response to variation in ambient temperature, there
was variation in per day distillate collected. The distillate pro-
duction rates of the three stills are presented in terms of the
scaled area of 0.6 m2 and normalised area of 1.0 m2. In the con-
ventional still, total distillate collected was very low compared to
the modified stills. When the ambient temperature was 32°C the
distillate collected was 2670 ml/m2d. The production reached
the maximum of 3670 ml/m2d when the ambient temperature
was 38°C. When heat storage materials were added in the basin,
there was improvement in the production. The minimum and
maximum production recorded were 3000 and 4670 ml/m2d
respectively. When external reflector was used to focus addi-
tional solar radiation into the still there was significant improve-
ment in the distillate production. The maximum production
touched 6000 ml/m2d and the minimum production dipped to
4000 ml/m2d.
7. 6 S. J. GNANARAJ AND V. VELMURUGAN
Table 2. Performance of the stills – comparisons.
Distillate output
Conventional still
Still with heat
storage materials
Still with
Reflectors
S.No Date
Maximum ambient
temperature (°C)
Scaled area
(0.6 m2)
ml/d
Normalised
area (1 m2)
ml/m2d
Scaled area
(0.6 m2)
ml/d
Normalised
area (1 m2)
ml/m2d
Scaled area
(0.6 m2)
ml/d
Normalised
area (1 m2)
ml/m2d
1. 29.03.2016 34 1700 2830 2100 3500 2800 4670
2. 04.04.2016 35 1900 3170 2400 4000 3300 5500
3. 09.04.2016 32 1600 2670 1900 3170 2400 4000
4. 13.04.2016 33 1600 2670 1800 3000 2500 4170
5. 16.04.2016 37 2100 3500 2600 4330 3400 5670
6. 19.04.2016 38 2200 3670 2800 4670 3600 6000
7. 21.04.2016 34 1700 2830 2200 3670 2900 4830
Avg. 1828 3048.57 2257 3762.86 2985 4977.14
Figure 6. Per day distillate production.
Figure 7. Increase in productivity.
The maximum production and the minimum production col-
lected during the course of the experiment and the average for
the seven days production is shown in Figure 6.
The difference between the maximum production and the
minimum production was caused by environmental forces. The
average of seven days production gives per day production of
the still. The average distillate collected from the conventional
still, still with heat storage materials and still with reflector was
3049, 3763 and 4977 ml/m2d respectively.
The percentage increase in the productivity of the modified
stills over conventional still is given in Figure 7.
Figure 8. Basin water temperature.
The productivity of the still with heat storage material was
23.08% higher than the conventional still. But the productivity
rise in the still with reflector was 62.97%.
To conclude, the distillate yield of the single basin double
slope solar still remarkably improved when it was modified by
using heat storage materials or external reflectors.
4.2. Impact of modifications on the temperature
parameters
The ambient temperature, basin water temperature and glass
cover temperature were recorded for every one hour. The
changes in basin water temperature and glass cover tempera-
ture differed from still to still and the modifications incorporated
in the still determined the pattern of change.
4.2.1. Basin water temperature
The main purpose of incorporating modifications in the solar
still is to enhance the basin water temperature and to sustain
it for a longer period. The still that maintains higher water tem-
perature produces more yield. The pattern of change in water
temperature is given in Figure 8.
From the above figure, it is clear that the peak level water
temperature attained differed from still to still. Further the
rate of change of water temperature also differed from still
to still.
8. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 7
4.2.1.1. Peak level water temperature. In the conventional
still and still with reflector, the basin water temperature crossed
50°C at 12’O clock itself and remained above this level upto 3
pm But the still with heat storage materials crossed this mark at
1 pm only. But it remained above this level upto 4 pm Among
the three stills, the still with reflector recorded the highest water
temperature of 63°C at 1 pm In the still with heat storage mate-
rials and the conventional still, the peak level water temperature
was 59°C and 56°C respectively and they touched this level at 2
pm only.
From the above it is clear that the modified stills had higher
peaklevelwatertemperaturecomparedtotheconventionalstill.
Among the modified stills, the still with reflector scaled a higher
peak level water temperature than the still with heat storage
materials.
4.2.1.2. Rate of change of water temperature. In the
forenoon, the basin water temperature slowly increased and
reached the maximum at noon. In the afternoon, there was a
decline in temperature. But the rate of increase and decrease in
basin water temperature differed from still to still and the modi-
fications incorporated in the still determined the difference.
When the water temperature was increasing, the rate of
increase was slow in the still with heat storage materials and fast
in the still with reflector. In the still with heat storage material,
the basin water along with basin materials received the solar
radiation. The basic characteristics of the heat storage materi-
als are that they absorb the excess heat energy received during
the bright sunshine hours. As a result, the increase in water tem-
perature was slow in the forenoon. On the other hand, the basin
water in the still with reflector received additional solar radiation
in the forenoon with the help of reflector. This resulted in rapid
rise in water temperature.
When the water temperature was declining, a different trend
prevailed. In the still with reflector, there was rapid decline in
water temperature. In contrast to this, in the still with heat stor-
age materials, there was a slow and gradual decline in water
temperature. In the afternoon, as the intensity of solar radia-
tion declined, the additional solar radiation focused into the still
declined. As a consequence, the still with reflector experienced
fast drop in water temperature. On the other hand, in the still
with heat storage materials, the basin materials started releasing
the stored up heat energy to the basin water. As a result, the
basin water in the still remained at a higher temperature com-
pared to other stills where the solar radiation intensity was
declining.
To conclude, the rate of increase or decrease of water tem-
perature was slow in the still with heat storage materials. On
the other hand, in the still with reflector, the rate of increase or
decrease of water temperature was rapid.
4.2.2. Impact of modifications on water-glass temperature
difference
Wider water-glass temperature difference accelerates evapo-
ration and condensation and enhances the distillate yield. So
the modifications incorporated in the conventional still must
widen the temperature difference. Water-glass temperature that
prevailed in stills at the end of every one hour duration is shown
in Figure 9.
Figure 9. Water-glass water temperature difference.
Figure 10. Distillate production rate.
In the still with heat storage materials, the water glass tem-
perature difference was almost equal to the conventional still
upto 12’O clock. After 12’O clock, it maintained a higher tem-
perature difference upto 6 pm But the still with reflector started
maintaining a higher water glass temperature difference from
the beginning of the experiment and this trend continued upto
2 pm After that, the temperature difference was almost equal to
that of the conventional still. It is better to make a comparison
between the two modified stills. Compared to the still with heat
storage materials, the still with reflector had higher water-glass
temperature difference upto 2 pm After 2 pm, the temperature
difference of the still with heat storage materials surpassed the
temperature difference of the still with reflector.
To conclude compared to the conventional still, the modified
stills maintained a higher water-glass temperature difference.
Among the modified stills, the still with reflector, had higher
temperature difference in the increasing sunshine hours and the
still with heat storage materials during the declining sunshine
hours.
4.3. Impact of modifications on the distillate production
rate
The distillate production rate varied from still to still and from
time to time and the variation is shown in Figure 10.
9. 8 S. J. GNANARAJ AND V. VELMURUGAN
Figure 11. Distribution of production.
In the still with heat storage materials, the increase in pro-
duction rate was slow compared to the conventional still in the
forenoon. In the afternoon, the production rate was consistently
higher. But in the still with reflector, the production rate was
higher than the conventional still throughout the experiment.
It will be useful to make a comparison between the two modi-
fied stills. Compared to the still with heat storage materials, the
production rate was higher in the still with reflector upto 2 pm
After that the production rate of the still with heat storage mate-
rials surpassed the production rate of the still with reflector. In
the conventional still, the production rate exceeded 650 ml/m2h
around 1 and 2 pm only. But the production rate was above
650 ml/m2h between 1 pm and 4 pm in the still with heat stor-
age materials and between 12’O clock and 3 pm in the still with
reflector. The still with heat storage materials and the still with
reflector recorded the peak per hour production of 900 ml/m2h
and 950 ml/m2h respectively at 2 pm
To conclude, the reflector was helpful in accelerating pro-
duction rate during the bright sunshine hours and heat stor-
age material was helpful in sustaining production during the
declining sunshine hours.
4.4. Impact of modifications on the distribution of
production during the day
For a better understanding of the pattern of distillate produc-
tion, the experiment day is divided into 4 quarters – upto 9 am,
9 am to 12’O clock, 12’O clock to 3 pm and 3 pm to 6 pm and the
distribution of production during the above 4 periods is given in
Figure 11.
In the first three quarters, the quantum of production in the
still with reflector was higher than the other stills. In the last
quarter, the volume of production in the still with heat stor-
age materials was higher than the other stills. When a com-
parison was made between the conventional still and the still
with heat storage materials, the quantity of production in the
former still was higher in the first two quarters and in the lat-
ter still in the last two quarters. But compared to the conven-
tional still, the still with reflector produced higher yield during
all the 4 quarters. Compared to still with heat storage materials,
the production of the still with reflector was marginally higher
during the third quarter and significantly lower during the fourth
quarter.
Figure 12. Deviation in production over conventional still.
4.5. Comparison of the production rate of the modified
stills with the conventional still.
A study on the pattern of increase or decrease in the per hour
distillate production of the modified stills over the conventional
still during the course of the experiment is helpful to ascertain
the nature of modification to be incorporated in the conven-
tional still. An increase in production over the conventional still
is shown in the positive side and the decrease in the negative
side of Figure 12.
The production of the still with heat storage materials was
marginally lower than the production of the conventional still
upto 12’O clock. After that it started producing more than the
conventional still. The excess of production over the conven-
tional still increased at an increasing rate in the afternoon and it
continuedupto4pmAfterthattherewasagradualdeclineinthe
excess production rate. There was excess production in the still
with reflector over the conventional still throughout the course
of the experiment. As the solar intensity increased the quantum
of excess production started increasing and it continued upto
12’O clock. After that the rate of increase of excess production
started declining.
To conclude, to get excess production over the conventional
still in the forenoon, the still has to be modified with reflectors.
For sustaining excess production in the afternoon, heat storage
material is very helpful.
4.6. Experimental uncertainty (U)
The experimental uncertainty (Internal + External) is calculated
for the most sensitive parameter, i.e. distillate water. It is calcu-
lated using the equation used by Bhupendra Gupta, Kumar, and
Table 3. Accuracy of various measuring instruments.
S.no Instrument Accuracy Range Error (%)
1. Thermometer ±1°c 0–100°C 5
2. Thermocouple ±0.1°c 0–100°C 0.5
3. Kipp-Zonen Solarimeter ±1 W/m2 0–2500 W/m2 2.5
4. Measuring jar ±10 ml 0–1000 ml 10
10. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 9
Table 4. Comparison of present work with previous research works.
SL.No Author(s) Modification
Productivity increase over
the conventional still
1. Abdullah (2013) Aluminium filling 53%
2. Panchal (2015) Black granite only 9%
3. Rajaseenivasan et al. (2016) River sand, metal scrap and charcoal as basin material 26.74%, 29.3% and 33.7% respectively
4. Srithar et al. (2016) Fins filled with river sand and charcoal 34.1% and 25.6% respectively
5. Madhua et al. (2018) Rubber mat and polyester mat 57.1% and 59.5% respectively
6. Omara, Kabeel, and Younes (2014) Both internal and external mirror 125%
7. Omara, Kabeel, and Younes (2013) Internal mirror only 75%
Present study (a) Heat storage material only 23.08% & 62.97%
(b) External reflector only
Baredar (2017).
U =
σ2 + σ2 + σ2 + · · · + σ2
n
N
σ = standard deviation
N = number of observations
σ =
(X − ¯X)
2
N
(X − ¯X)2 = Deviation from the mean
Percentage of internal uncertainty = U
mean of observation
The external uncertainty is taken as the least count of the
measuring instruments. The experimental uncertainty of the
conventional still, still with heat storage materials and still with
reflector was 13.29%, 16.15% and 15.4% respectively. The per-
centage of uncertainty is within the acceptable range.
4.7. Error analysis
Error associated with the experimental measurement apparatus
suchasthermocouple,solarimeter,thermometerandmeasuring
jars are shown in Table 3 and they are calculated as fallows.
Error =
Accuracy of the instument
Minimum value of the output measured
× 100
4.8. Comparison of the performances of solar stills
A comparison of the performance of the present work with
previous works on solar stills is given in Table 4.
Figure 13. Comparison of experimental and theoretical production values.
4.9. Comparison of experimental and theoretical
production values
A comparison of the experimental production values obtained
after conducting the experiment with the theoretical values
derived from calculation is necessary to ascertain the effi-
ciency achieved by the stills constructed. A comparison between
the theoretical and experimental distillate production values is
given in Figure 13.
There was close agreement between the theoretical and
experimental values. Experimental values were marginally lower
than the theoretical values.
5. Conclusion
The productivity of the single basin double slope solar still was
very low. An attempt was made to enhance the productivity of
the still by spreading heat storage materials in the basin or by
using external reflector. The performance of the conventional
still was compared with the performance of the modified solar
stills. The following inferences were drawn.
1. The productivity of the still with heat storage materials was
23.08% higher than the conventional still. But the produc-
tivity rise in the still with reflector was 62.97%.
2. In the still with reflector, the increase in basin water temper-
ature was quick in the forenoon and the peak level temper-
ature of 63°C was attained at 1 pm itself. Compared to the
still with heat storage material, it maintained higher water
temperature upto 2 pm As the sunshine declined, there was
a drastic fall in water temperature.
3. In the still with heat storage material, there was a slow
increase in basin water temperature in the forenoon and
the maximum temperature of 59°C was touched at 2 pm
only.After2pm,inthedecliningsunshinehours,itsustained
higher water temperature compared to the still with reflec-
tor. The heat storage materials delayed quick drop in water
temperature.
4. In the conventional still, the per hour distillate production
was about 650 ml/m2h at 1 and 2 pm The distillate produc-
tion was more than 650 ml/m2h between 1 and 4 pm in still
with heat storage materials and between 12’O clock and 3
pm in the still with reflector. Still with heat storage materials
and still with reflector produced the peak level production
of 900 and 950 ml/m2h respectively at 2 pm
5. Reflector was helpful in accelerating production rate dur-
ing bright sunshine hours and heat storage material was
11. 10 S. J. GNANARAJ AND V. VELMURUGAN
effective in sustaining higher production rate during the
declining sunshine hours
6. A study on the production pattern during the four quarters
of the experiment period revealed that the still with reflec-
tor had higher production during the first three quarters.
During the last quarter, the still with heat storage material
recorded higher production.
To conclude, modification in the single basin double slope
solar still was really effective in enhancing the productivity of
the still. For boosting forenoon production, external reflector
was helpful and for sustaining higher production rate in the
afternoon, the heat storage material was helpful. Modifying the
conventional still with heat storage material and reflector will
guarantee substantially higher distillate production throughout
the day.
Disclosure statement
No potential conflict of interest was reported by the authors.
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